The present invention relates to an improved blowout preventer with improved tubing shear rams.
Prior to the present invention blowout preventers have been provided with tubing shear rams, but they were sized to shear a particular size of tubing and they functioned to shear the tubing string so that the upper end of the tubing left in the well bore was flattened and in subsequent operations, in order to be recovered by a fishing operation, had to be cut or otherwise manipulated so that the upper end was open and so that the overshot could properly engage and recover the string.
U.S. Pat. Nos. 4,132,266; 4,132,267; 4,341,264 and 4,531,585 are typical examples of such prior tubing shear rams. The rams flattened, bent and closed the upper end of the tubing string left in the well bore. Additionally, the blowout preventer was provided with shear rams which were sized to shear a particular size tubing but did not always function properly when shearing smaller or larger tubing strings. U.S. Pat. No. 4,081,027 discloses another type of blowout preventer with shear blades, and the upper end of the lower fish is closed by the shearing action as is clearly shown in FIG. 4 of this patent. Further, U.S. Pat. No. 4,240,503 discloses a shearing type of blowout preventer with the sealing after cutting being by a seal strip under the upper blade which, when the blades are closed, is caused by the flow of the elastomer responsive to such closing to move into sealing engagement with the upper surface of the lower blade.
U.S. Pat. No. 4,537,250 discloses a blowout preventer which includes shearing blades with a node or nodes on the lower blade to reduce the shearing force. Also, this patent discloses the use of a concave blade shape to support the string during shearing sufficiently to constrain the string below the upper shear blade as it is sheared to a shape suitable for receiving an overshot type of retrieving tool and to allow flow therein.